In traditional circuit-switched voice networks, a basic feature of many private branch exchanges (PBXs) is music-on-hold (MOH). A caller may be placed on hold either by a user of a handset connected to the PBX or automatically (e.g. while queuing for an operator to answer a call). In either case, the PBX switches MOH onto the voice channel as soon as the caller is placed on hold, so that music is delivered to the caller while he or she is holding.
The source of the MOH may, for example, be a disk or tape drive connected to the PBX, or a radio tuner which delivers the content of a radio station to the caller, or it may be electronically generated music from the PBX. In any case, this music is delivered in either digital or analog uncompressed format with minimal distortion to the caller who is placed on hold.
With packet-based voice networks, valuable Wide Area Network (WAN) resources are used whenever a voice call is made. Voice signals are therefore normally compressed by means of an algorithm known as a codec, before being transmitted as packets across the network, to reduce bandwidth.
The International Telecommunication Union defines a number of standard codecs which are used to compress telephony signals. Two examples of such codecs which are used in Voice over Internet Protocol (VoIP) calls are G.729AB and G.723.1, which result in data transfer rates of 8 kb/s and 6.4/5.333 kb/s respectively. Both codecs provide acceptable standards of voice quality but since they are not designed to work on non-voice inputs, such as music, they often give unacceptable results when so used. This has the result that when used with common codec standards, MOH delivered to the far-end PBX (where the caller on-hold is located) will be rendered unrecognisable by the encoding, transmission and decoding.
While there are uncompressed codecs (e.g. G.711 A Law) available for packet-based networks, it is wasteful of bandwidth to use these codecs for the transmission of MOH due to the bandwidth-imposed restrictions of the networks.
In applications such as video telephony, the bandwidth requirements will in general be greater, and it is therefore even less desirable to deliver video content to callers who are on hold, even though this might be a desirable feature for customers of video conferencing solutions.
It is therefore an object of the present invention to provide improved quality media content to callers on hold.